1. Field of the Invention
The present invention relates to creation of inverse synthetic aperture radar (ISAR) images, and in particular to an enhanced auto-focusing technique for ISAR imaging.
2. Background of the Development of SAR/ISAR Imaging
Doppler-frequency radar has been used to produce map images since the early 1950's. The earliest Doppler radar imaging system did not provide phase correction to compensate for changing Doppler frequencies due to relative motion between the radar platform and the target. Such a system is said to be "unfocused."
Synthetic aperture radar (SAR) systems evolved from this early Doppler frequency analysis mapping. In SAR systems the relative motion between the radar platform and the target area can produce an apparent rotation rate of the target. For example, if an aircraft is over a strip of terrain, prominent features of the terrain will produce variations in the returned signal. The most prominent features of the target that provide radar return signals that are used for mapping are called "scatterers."
The Doppler frequency signal produced by a scatterer is a function of time as the aircraft traverses the terrain strip. A train of coded pulses is transmitted by the Doppler radar. SAR cross-range resolution (i.e., resolution perpendicular to the line-of-sight from the platform to the scatters) is much finer than is attainable from the radar's beam width. Motion induced frequency changes are corrected by determining a phase-shift factor that is applied to modify stored phase information of the return signals. Such correction is called "focusing" the synthetic aperture.
SAR systems are best suited to applications where there is no target motion since target motion may seriously degrade SAR images. Inverse synthetic aperture radar (ISAR) was developed to produce Doppler radar images of moving targets. With ISAR, images of scatterers on the target are resolved in the range direction by use of large bandwidth waveforms and in the cross-range direction by integration of the return signals. ISAR imaging has been proposed to form images of targets which have no appreciable translational motion relative to the transmitting radar but which may be yawing, pitching or rolling, such as a ship at sea.
Ideally, for ISAR imaging, the radar and the target should have no relative translational motion. Then, as the target rotates, scatterers on the target will provide a Doppler frequency which is dependent upon the distance of a scatterer from the center of rotation of the target, the angular rate of rotation of the target, the transmitted wavelength and the angle between the line-of-sight to the target center of rotation and a line that joins the scatterer and the center of rotation. Thus, the same scatterer at various points of the rotation will provide a return signal of a varying Doppler frequency.
Cross-range resolution is dependent upon how well the frequency difference between the various scatterers can be resolved. In ISAR imaging the relative motion of the target is usually uncontrollable. Therefore, the signal processor system associated with the radar has to estimate the target translation and rotation with respect to the radar platform to make the necessary corrections for adequate resolution of the image. The ISAR process maps a three-dimensional target into a two-dimensional image which depends upon the target's rotational motions.
In a conventional strip-map mode SAR, range resolution is achieved through accurate time delay measurements which are obtained by transmitting frequency-dispersed pulses and applying pulse compression techniques to the returned pulses. This is described on page 379 of an article entitled "Developments in Radar Imaging" by Dale Ausherman, et al., in the IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-20 No. 4, July 1984. A more sophisticated multiple sub-patch processing technique is described on pages 384-385 of the same article, especially with reference (on pages 385 and 387) to FIGS. 22 and FIGS. 27, respectively.
In this system an image swath is divided into a number of image sub-patches which break up the images into small areas which are sequentially illuminated by the radar antenna as the platform moves past the target. Motion of each of these sub-patches is compensated for relative to their respective centers to provide a reference function which changes on a pulse-by-pulse basis according to pulse-by-pulse range changes. The signal from each of these sub-patches is then sent to bandpass filters which filter the signal into a number of frequency sub-bands. Frequency sub-bands are provided which correspond to N range sub-swaths across M bandpass filters. Each of these N sub-swaths are partitioned into M sub-bands which correspond to the multiple sub-patches in the cross-range direction. The data within each channel is two-dimensionally Fourier-transformed to produce the sub-patch images which are then combined to form a full-scan image.
U.S. Pat. No. 4,546,355, issued Oct. 8, 1985, entitled "Range/Azimuth/Elevation Ship Imaging for Ordinance Control" issued to Sol Boles, describes a method of processing Doppler interferrometric azimuth and elevation angle information obtained from the reflection or radar signals from scatterers on a ship which are combined into a weighted multi-variant regression fit using digital signal processing techniques. These techniques provide measures of ship translational and rotational motions for focused high resolution imagery. The patent also describes the capability of scaling a cross-range Doppler dimension of ISAR profiles. The "scaled" or "stretched" ISAR images which have their ordinate values derived directly from Doppler representation rather than values derived from elevation angle measurements are also discussed in column 9 of this patent. A Fast Fourier Transform digital waveform produces Doppler resolved coherently integrated vector sums which are used for range/Doppler map generation and range/azimuth map generation.
United States Statutory Invention Registration H955, published Aug. 6, 1991 in the name of William M. Walters, describes an ISAR imaging technique which utilizes a primary antenna, and an auxiliary antenna to estimate aircraft altitude rate.
United States Statutory Invention Registration H910 published Apr. 2, 1991, entitled "Adaptive Two-Dimensional Shading for Batch Synthetic Aperture Using Phased Radar Antenna" published in the name of Lawrence J. Hindenach, describes a SAR processing system in which a number of sample points in the frequency range domain, for which the gain estimation is to be made, are selected. These are divided into a plurality of sample points and overlapping groups. Transformation is then made from frequency range domain to antenna coordinates.
U.S. Pat. 4,855,747, issued Aug. 8, 1989, entitled "Method of Target Imaging and Identification" which issued in the name of Bernard D. Steinberg, describes the technique of combining ISAR imaging with adaptive beam-forming.
U.S. Pat. 4,616,227, issued Oct. 7, 1986 in the name of Koichi Honma, et al., entitled "Method of Reconstructing Synthetic Aperture Radar Image" describes a method of re-sampling position designation data from Doppler rate-times-change data and re-sampling the image, after range compression in the azimuth direction, using the azimuth coordinates and transformation data in order to keep unaltered the Doppler rate of a point image relating to the image after record transformation.
Prior SAR and ISAR Imaging Error Correction Techniques
When a platform carrying a Doppler radar and using ISAR imaging processing moves relative to a target, such as a ship in a heavy sea, the line of sight angle between the platform, which may be an airplane, and the ship will change as the airplane first approaches and then recedes from the ship. There are several sources of errors which occur during ISAR imaging. One such source of error is termed the quadratic phase error which occurs because the angle between the path of the plane and the target point varies as the range between the plane and the target ship changes.
With reference to FIG. 1, for example, at range R1, the angle between the flight path and the path of the radar beam is .THETA.1, whereas at range R2, when the plane is closer to the target ship, the line of sight angle is 2, where the angle .THETA.2 is larger than the angle .THETA.l. This change in the line-of-sight angle results in blurring of the ISAR imaging due to slight variations over the integration interval. It is the primary source of error that also occurs in the traditional SAR imaging of terrain. This type of error is well understood and is removed from SAR and ISAR images by using traditional auto-focus techniques which are well known to those engaged in the arts of SAR and ISAR. In SAR as well as in ISAR systems, a continuous pulse train is sent from the platform to the target, and the returning wave signals are sampled on a constant time difference basis, and correction is made in the quadratic phase errors that are caused by range variations. This correction is made by using time-delay processing techniques which normalize the distance between the Doppler frequency platform and the target.
SAR imaging was initially used for ground-mapping and to provide images of stationary targets. It was later realized that the same Doppler radar principles could be applied to imaging of moving targets. Of course, this introduced more complications and additional errors which must be corrected in order to obtain satisfactory images. Higher order phase errors occur when there is relative acceleration between the platforms, such as an airplane and a target ship where target ship accelerations are created by the heave, surge and sway wave action of the ocean, as illustrated in FIG. 2.
The type of errors that have been previously corrected in ISAR systems have been those which occur when the acceleration of the target is relatively constant over a sampling period of time. Phase errors caused by relatively constant accelerations have been removed from ISAR signals in the past by use of multiple sub-aperture processing in which the radar data obtained for each sub-aperture is processed by an FFT algorithm or device to provide amplitude versus frequency sub-images, each of which corresponds to a sub-aperture, which is used for error correction processing.
ISAR, prior to the disclosed invention, was not totally satisfactory, however, for providing imaging of moving targets and in particular for imaging surface ships at sea, even when the range-dependent quadratic phase errors and the higher order acceleration phase errors had been compensated by the above-described prior techniques because of pitch and roll rotational acceleration variations caused by waves.
FIG. 5a shows a ship with a rolling action that is in a position where it is rotating at a maximum rotation rate and, therefore, has a zero rotational acceleration. FIG. 5b shows the same ship at the end of the roll cycle where the ship's rotational velocity is zero and the rotational acceleration is a maximum. The type of error created by such rotation where the rotational acceleration rate varies is a frequency-dependent error. Prior art ISAR image correction algorithms have assumed that the ship is undergoing rotation at an approximately constant rate of rotation so that the pulses, which are sampled at constant time-difference intervals, correspond to a constant angular-difference sampling. In actuality, the rotation rate of the ship will vary over the time used for image formation so that a correction is necessary to obtain constant angular-difference samples from the constant time-difference pulses.
ISAR signals caused by time-varying rotation are not only time-dependent, but they also are frequency-dependent. Prior correction techniques do not remove these rotational artifacts from ISAR images, and this substantially limits the quality of ISAR images of ships at sea and of other targets or applications that require high resolution imaging of targets that are capable of undergoing rotational acceleration, particularly variable rotational acceleration.